Over the last few months, I have been topping up the pressure on one of the tyres but only every 3 or 4 weeks. The others have kept their pressure.
Yesterday it suddenly lost all pressure whilst driving (not too far from home).
The guys at the tyre centre (a well trusted independent firm) said the rim was corroded and that was causing a gradual leak. They tried their best to get rid of the corrosion but it has lost pressure again really quickly.